Assemblies, polymers, crystals, and biological structures bearing nanochannels have been studied for many decades. Examples of nanochannel materials are zeolites, zeotypes, cyclodextrins, urea based assemblies, mesoporous silica materials, collagens, metal organic frameworks, as well as organic-, carbon- and metaloxide-channels. All have been investigated to some extent as hosts for molecules, complexes, ions, or clusters. One dimensional (1D) nanochannels have special properties which have recently attracted considerable attention. We distinguish between several types of 1D channels: single file, ordered assemblies, amorphous, “semicrystalline”, and crystalline. A common feature of 1D channels is that they have only 2 entrances. Channels can therefore be open on both sides, plugged on one side, or plugged on both sides. This plugging can be isolating or it can be partial, allowing electrons, protons, or small molecules to pass, but blocking larger objects. Zeolites with hexagonal structures are interesting materials bearing 1D channels and among them zeolite L (ZL) is especially important for preparing strongly luminescent organic-inorganic dye-zeolite hybrid materials. (U.S. Pat. No. 7,914,702; G. Calzaferri et al.: Mimicking the Antenna System of Green Plants, Photochem. Photobiol. Sci., 2008, 7, 879; D. Brühwiler et al.: Nanochannels for Supramolecular Organization of Luminescent Guests, J. Mater. Chem., 2009, 19, 8040; EP 18732002, U.S. Pat. No. 7,655,300; WO 2008/052603, PCT/EP2007/005811, China 1128455; G. Calzaferri et al.: Designing Dye-Nanochannel Antenna Hybrid Materials for Light Harvesting, Transport and Trapping, ChemPhysChem, 2011, 12, 580.)
Classical zeolites are aluminosilicate crystalline minerals which have a microporous structure, according to IUPAC nomenclature, but which are actually nanoporous. Their “open” structure can accommodate a wide variety of molecules, ions and clusters. Zeolites are therefore often referred to as “molecular sieves”. The term molecular sieve refers to a particular property of zeolites, i.e. their ability to selectively sort molecules based primarily on a size exclusion process. The size exclusion process is due to the highly regular pore structure of molecular dimensions. The maximum size of a molecular or ionic substance that can enter the pores of the zeolite is controlled by the diameters of the channels within the zeolite. These channels are conventionally defined by the ring size of the aperture, where, for example, the term “12-ring” refers to a closed loop that is built from 12 tetrahedrally coordinated silicon or aluminum atoms and 12 oxygen atoms. ZL crystals, which can be prepared in a size range of 30 nm up to about 10000 nm and with different morphology such as elongated, barrel type and disc shaped crystals, have hexagonal structure. A ZL crystal of 600 nm diameter consists of roughly 100000 strictly parallel channels, with a channel opening diameter of 0.71 nm and a largest inner diameter of 1.24 nm, which can be filled e.g. with dye molecules. The ZeoFRET® (Zeo=zeolite, FRET=Förster Resonance Energy Transfer) nanochannel-materials are highly organized dye-zeolite inclusion compounds with photonic antenna function. After absorption of the incident light by high local concentrations of dye molecules, the energy is transported by FRET to an acceptor A. High donor-to-acceptor (D:A) ratios and multi-donor systems are promising as active species in luminescence concentrators (LC) and luminescence solar concentrators (LSC) (WO 2010/009560) High D:A ratios thereby open possibilities to reduce self-absorption, also known as inner filter effect, while maintaining efficient light-harvesting. We have synthesized ZeoFRET® materials according to this concept with different D:A ratios. In one example, one channel of ZL contained approximately 150 donor dyes and on average 1.5 acceptor dyes at each channel end (G. Calzaferri et al.: Designing Dye-Nanochannel Antenna Hybrid Materials for Light Harvesting, Transport and Trapping, ChemPhysChem, 2011, 12, 580.)
The packing of the molecules inside the channels influences the materials properties. So called H-dimers can be formed if the molecules are sufficiently small. This patent focuses on molecules too large and channels too narrow for allowing this. The molecules may, however, still be of a size and shape so that they can come close enough for allowing sufficiently strong coupling of their electronic transition dipole moments thereby increasing the importance of excimer and exciton states. Signature of Davydov coupling (herein called J-coupling) has therefore been observed, studied, and reported. J-coupling strength, which we abbreviate as βc, can be calculated as follows:βc=AD·f·κ/(ΔE·R3·n2)The value of the constant AD is equal to 1.615·10−18 m2 cm−1 if we express βc in cm−1. The magnitude of interactions βc caused by exciton coupling, and hence the resulting splitting of the levels, depends on the oscillator strength f, the relative Orientation κ of two neighboring electronic transition moments, and the distance R between the interacting electronic transition dipole moments. The expression for κ can be simplified in the present case as κ=1−3 cos2(θ), where θ is the angle between two adjacent electronic transition dipole moments with values close to 0° and always smaller than 40°. A typical orientation of the electronic transition moments of the chromophores for the materials of interest is shown in FIG. 1 as double arrows. This distance is very often equal to the distance between the centers of the involved chromophores. βc further depends on the electronic excitation energy ΔE, and on the refractive index n of the environment. (G. Calzaferri et al.: Mimicking the Antenna System of Green Plants, Photochem. Photobiol. Sci., 2008, 7, 879; M. Busby et al.: Time, Space and Spectrally Resolved Studies on J-Aggregate Interactions in Zeolite-L Nanochannels, J. Am. Chem. Soc., 2008, 130, 10970; M. Busby et al.: Interactions of Perylene Bisimide in the One Dimensional Channels of Zeolite L, J. Phys. Chem. C, 2011, 115, 5974; UK 0812218.6, US 2010/0003188).
High D:A ratios are needed for efficient light-harvesting of the ZeoFRET® materials, the technical applications of which have been described in the references given above. The donors D absorb light at shorter wavelengths than the acceptors A. In order to shift the light absorption of the material to longer wavelengths, donors must be chosen such that they absorb at longer wavelength which means that the acceptor absorption wavelength must be shifted accordingly. This is not an important problem as long as the absorption wavelength threshold of the ZeoFRET® is not larger than 550 nm or 600 nm. It causes, however, practical problems at longer wavelengths which increase the more the absorption wavelength is shifted to large values such as 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, or even 950 nm. One of the reasons for these problems is the limited number of molecules available that can act as good acceptors, that are sufficiently stable, that have large luminescence quantum yield, and that can be inserted into the channels of the host material used for the synthesis of ZeoFRET® in a way that a photochemically and thermally stable ZeoFRET® material with large luminescence output can be synthesized. An important reason for near infrared (NIR) absorption and emission is the fact that skin and muscles are sufficiently transparent in the wavelength range of 650-900 nm, which is known as (optical) therapeutic window for e.g. a detection or action depth of 500 micrometer up to a few cm (R. Weissleder: A Clearer Vision for in vivo Imaging; Progress Continues in the Development of Smaller, More Penetrable Probes for Biological Imaging, Nat. Biotechnol., 2011, 19, 316.)